1. Field of the Invention
The present invention relates to a control device for a PWM controlled converter which converts an AC power source voltage into a DC voltage.
2. Description of the Related Art
There are many devices which convert AC power to DC and the number of such devices is increasing. In this type of device, an AC power source voltage is converted into a DC voltage, and the converted DC voltage is used. In the AC to DC converting operation, reactive power and harmonics are generated, which causes some problems. To solve these problems, a PWM controlled converter has been proposed as a device for converting an AC power source voltage into a DC voltage.
FIG. 28 is a block diagram showing a conventional control device for a PWM controlled converter. The control device for a PWM controlled converter is as discussed in "Method for Controlling a P-phase Invertor not Using an Insulating Transformer", 1991 National Convention Record IEEE Japan, pp 5 to 80, or as disclosed in Japanese Patent Laid-Open Publication No. Hei. 3-212162.
In the figure, reference numeral 1 designates a 3-phase AC power source; numeral 2, a PWM controlled converter which converts AC power received from the 3-phase AC power source 1 into a DC voltage-by controlling the AC current of the received AC power, and outputs the converted DC voltage; numerals 201 to 206 designate switching elements which are transistors, IGBTs (insulated gate bipolar transistors) and the like; and 207 to 212 designate circulating diodes. Numeral 3 designates reactors connected between the 3-phase AC power source 1 and the PWM controlled converter 2; 4 designates a smoothing condenser for absorbing a pulsating component contained in an output current of the PWM controlled converter 2; 5 designates a load unit, such as an invertor or a resistive component; 6 designates a voltage setter for outputting a voltage setting signal for setting a DC voltage output from the PWM controlled converter 2; and 7b designates a voltage detecting circuit for detecting a DC voltage output from the PWM controlled converter 2.
Numeral 8 indicates a subtractor for calculating a deviation of a voltage detecting signal detected by and output from the voltage detecting circuit 7b from a voltage setting signal set by and output from the voltage setter 6; 9d designates a voltage controller which includes a proportional control calculating element and a proportional integration calculating element, and proportional integration (PI) controls a deviation of a voltage detecting signal output from the subtractor 8 from the voltage setting signal; 10 designates an AC voltage detector for detecting an AC voltage from the 3-phase AC power source 1; 11 designates a unit sine wave generator for generating R-phase and T-phase unit sine waves, which are synchronized with R-phase and T-phase voltages, from an AC voltage detecting signal detected by the AC voltage detector 10; 12 and 13 designate multipliers for multiplying a peak value instruction signal of an input current received from the voltage controller 9d by R-phase and T-phase unit sine wave signals received from the unit sine wave generator 11, and outputs R-phase and T-phase input current instruction signals.
Numerals 14 and 15 represent current detectors for detecting currents of R-phase and T-phase input to the PWM controlled converter 2; 16 and 17 represent subtractors for producing a deviation of an input current detecting signal of R-phase output from the current detector 14 from an input current instruction signal of R-phase output from the multiplier 12 and a deviation of an input current detecting signal of T-phase output from the current detector 15 from an input current instruction signal of T-phase output from the multiplier 13; 18b and 19b represent R- and T-phase current controllers which include proportional control calculating elements and proportional integration calculating elements, and proportional integration (PI) control the deviations of the T-phase input current detecting signals from the R-phase input current instruction signals, to thereby output an R-phase control signal and a T-phase control signal; 20 represents a subtractor for subtracting from zero the R- and T-phase control signals output from the R- and T-phase current controllers 18b and 19b; 21 represents a carrier wave oscillator; 22, 23 and 24 represent comparators for comparing the amplitudes of R-, S- and T-phase control signals with a carrier wave signal to output pulse-width modulated signals; 25 represents a gate circuit for outputting signals to turn on and off the switching elements 201 to 206 of the PWM controlled converter 2 in accordance with R-, S- and T-phase pulse-width modulated signals.
An operation of the conventional control device thus constructed will be described. A detecting value V DC of a DC voltage detected by the voltage detecting circuit 7b and a detecting value V DC* set by the voltage setter 6 are input to the subtractor 8 where the eV (deviation)=V DC -V DC* is calculated. The obtained deviation eV is input to the voltage controller 9d where it is PI-controlled to produce a peak value instruction signal I PEAK* of the input current. The peak value instruction signal I PEAK is applied to the multipliers 12 and 13 where it is multiplied by the unit sine wave signals of R-phase and T-phase which are other signals input to the multipliers. The R- and T-phase unit sine wave signals are reference AC signals synchronized with the R- and T-phase voltages of the 3-phase AC power source 1, and are generated by a current reference signal generator constituting the unit sine wave generator 11, which receives an AC voltage of the 3-phase AC power source 1 detected by the AC voltage detector 10. The multipliers 12 and 13 produce an R-phase input current instruction signal iR* and a T-phase input current instruction signal iT*, respectively.
The R-phase input current instruction signal iR*, which is an output signal of the multiplier 12, and an R-phase input current detecting signal iR, which is an output signal of the current detector 14, are applied to the subtractor 16 which calculates eiR (deviation)=iR*-iR and outputs the result of the calculation. Similarly, the T-phase input current instruction signal iT*, which is an output signal of the multiplier 13, and a T-phase input current detecting signal iT, which is an output signal of the current detector 15, are applied to the subtractor 17 which calculates eiT (deviation)=iT*-iT and outputs the result of the calculation. The current deviations eiR and eiT are applied to the R-phase current controller 18b and the T-phase current controller 19b, respectively. These current controllers PI-control the current deviations and output S- and T-phase control signals SR* and ST*, respectively.
An S-phase control signal SS* is produced in a manner that the subtractor 20 subtracts the R- and T-phase control signals SR* and ST* from zero. The R-, S- and T-phase control signals SR*, SS* and ST* as the output signals of the R-, S- and T-phase current controllers 18b, 20 and 19b, are applied to the comparators 22, 23 and 24, respectively. Those comparators compare the amplitudes of the control signals SR*, SS* and ST* with the amplitude of a triangle wave carrier signal, and produce pulse width modulation (PWM) signals. The PWM signals are applied to the gate circuit 25. The gate circuit outputs control signals so that a detecting value V DC of a DC voltage of the PWM controlled converter 2 is equal to a detecting value V DC*, and so that the R-, S- and T-phase input currents iR, iS and iT are equal to the R-, S- and T-phase input current instruction signals iR*, iS* and iT* as sine wave signals. In the PWM controlled converter 2, the switching elements 201 to 206 are turned on and off in accordance with the control signals received from the gate circuit.
In the thus constructed control device for a PWM control converter, as described above, the R- and T-phase current controllers 18b and 19b form a current control loop in which the R- and T-phase input current instruction signals iR* and iT* output from the multipliers 12 and 13 are used as instruction signals, and the R- and T-phase input current detecting signals iR and iT output from the current detectors 14 and 15 are used as feedback signals. The R- and T-phase current controllers 18b and 19b may be realized by a digital control technique using a microprocessor, for example, or an analog control technique using operation amplifiers, for example. When the digital control technique is used, sampling delays entail time lags. Therefore, in designing the current controllers, it is impossible to increase a response in the control system of the digital control basis current controller in excess of that of the analog control basis current controller. Therefore, in the digital control basis current controller, the following disadvantageous phenomena inevitably take place: on and off delays of the switching elements 201 to 206 in the PWM controlled converter 2, a deviation of an actual voltage that is caused by an on voltage from the voltage instruction value V DC*, a waveform distortion of an AC input voltage of the 3-phase AC power source 1, and the like. Accordingly, an AC input current waveform is not sinusoidal and contains harmonics defined by the waveform distortion.
To make the input current waveform faithfully trace a sinusoidal waveform, the analog control system that has no sampling delays, for example, and enables the current control system to have a high response is preferably used for the current controllers. Similarly, to make a DC voltage value in the PWM controlled converter 2 follow the set value, it is preferable to construct the voltage controller 9d on the basis of the analog control system which is free from the sampling delays, for example, and allows the voltage control system to have a high response.
FIG. 29 is a circuit diagram showing in detail the R- or T-phase current controller. In the circuit diagram, the R-phase current controller 18b as an IP controller which performs an analog control of current, and is realized by using an operational amplifier.
In FIG. 29, reference numerals 101 to 103 designate fixed resistors; numeral 104, a capacitor; 105, an operational amplifier; 106 and 107, positive and negative voltage input terminals of a control power source for driving the operational amplifier 105; 108, an input terminal; and 109, an output terminal. In the thus constructed R-phase current controller 18b, when an input signal at the input terminal 108 that is positive or negative continues for a fixed period of time or longer, the voltage across the capacitor 104, which provides an integration term of a proportional integration operation, increases in the positive or negative direction. However, it is possible for the capacitor voltage to increase to be below a positive voltage or above a negative voltage of the control power source applied through the positive and negative voltage input terminals 106 and 107. Thus, the capacitor voltage is saturated while being limited within a fixed value. Also, when an input signal coming in through the input terminal 108 is large, a signal outputted from the operational amplifier 105 after a signal amplifying process by the amplifier cannot vary to be below a positive voltage or above a negative voltage of the control power source applied through the positive and negative voltage input terminals 106 and 107. The output signal of the operational amplifier is saturated within a limited value.
The T-phase current controller 19b is also similarly constructed and operates in a similar manner. The circuit constructed as stated above is a basic circuit, generally used for realizing a proportional integration operation by using an operational amplifier.
FIG. 30 is a circuit diagram showing a specific example of the voltage detecting circuit 7b for detecting a DC voltage V DC. In FIG. 30, reference numeral 701 designates an input terminal connected to a positive potential of the smoothing condenser 4; and 702 and 703 designate fixed resistors for dividing a DC voltage V DC. The fixed resistor 703 is connected to a negative potential of the smoothing condenser 4. Numeral 704 designates an insulated amplifier; 705 and 706, fixed resistors; 707, an operational amplifier; and 708 and 709 designate variable resistors for adjusting an offset and a gain of the voltage detecting value. The thus constructed circuit is a basic circuit, which is generally used for adjusting offset and gain errors by an operational amplifier, and includes variable resistors for adjusting the offset and the gain errors.
In the conventional control device for a PWM controlled converter thus constructed, an overcurrent problem arises particularly when the control device is started up in a state that electric power has been supplied from the 3-phase AC power source 1 to the load unit 5.
In a case where electric power has been supplied from the 3-phase AC power source 1 to the load unit 5 in a state that the switching elements 201 to 203 of the PWM controlled converter 2 are in an off state, viz., before the control by the PWM controlled converter 2 starts and the gate circuit 25 is closed, the power is supplied to the load unit 5, through the reactors 3 and the circulating diodes 207 to 212. In this case, the waveforms of the R-, S- and T-phase input currents are as shown in FIG. 31.
In this case, the current flows through the reactors 3. Because of this, an input voltage of each phase to the PWM controlled converter 2 is lower than the voltage of the 3-phase AC power source 1 by a voltage corresponding to a voltage drop across the corresponding reactor 3. As a result, a DC voltage, or a voltage across the smoothing condenser 4, of the PWM controlled converter 2 is lowered.
When the controlling operation of the control device is started up from this state, the R- and T-phase current controllers 18b and 19b operate so as to compensate for a lowering of the DC voltage V DC. The R- and-T-phase control signals SR* and ST* output from those current controllers 18b and 19b increases substantially inversely proportional to the DC voltage V DC. In the conventional control device for a PWM controlled converter, the R- and T-control signals SR* and ST* are obtained through the calculations of the R- and T-phase current controllers 18b and 19b which perform PI operations, and the S-phase control signal SS* is obtained as SS*=(-SR*-ST*), which follows from a formula SR*+SS*+ST*=0. Since the current controllers operate so as to compensate for a lowering of the voltage V DC, when the R- and T-phase control signals SR* and ST* increase in the positive or negative direction and are saturated and fixed at a predetermined value, the S-phase control signal SS* is also fixed at a predetermined value. As a result, the control device fails to control the voltages of the three phases. Particularly, at the time of the above-mentioned starting up where the currents as shown in FIG. 31 flow, the current deviations as the input signals to the R- and T-phase current controllers 18b and 19b are connected to a positive or negative polarity for a fixed period of time. Therefore, a value of the integration term as a constituent element increases, so that the output signals of the R- and T-phase current controllers 18b and 19b are frequently saturated. As a result, the control device fails to control the voltages of three phases, an overcurrent flows in the control device and possibly drives an overcurrent protecting mechanism to trip. Incidentally, the overcurrent protecting mechanism is generally incorporated into the circuit for protecting circuit elements.
Also in a normal running state, when an electric power of the load unit 5 abruptly changes and the voltage V DC drops, the output signals of the R- and T-phase current controllers 18b and 19b are saturated as at the time of the above-mentioned starting up. The control device fails to control the voltages of three phases including the S-phase, an overcurrent flows in the control device, and possibly causes a trip of the protecting mechanism.
Also in a case where the voltage V DC is controlled as indicated by a set value, when the R- and T-current deviations eiR and eiT as the input signals to the R- and T-phase current controllers 18b and 19b become large as the result of an abrupt change of the current instruction signal, the proportional terms as the constituent elements of the R- and T-phase current controllers 18b and 19b become large, the output signals of the R- and T-phase current controllers 18b and 19b are saturated, and consequently the control device fails to control the voltages of three phases including the S-phase, an overcurrent flows in the control device, and possibly causes a trip of the mechanism.
In the control device for a PWM controlled converter, the analog control technique using the operational amplifier is widely used for constructing the voltage controller 9d since it is necessary to precisely control the DC voltage that appears at the output of the PWM controlled converter 2. The voltage controller 9d receives a deviation eV of a detecting value V DC detected by the voltage detecting circuit 7b from a set value V DC* set by the voltage setter 6; eV=V DC*-V DC . Then, it PI controls the deviation eV to produce a peak value instruction signal I PEAK* of the input current. Therefore, the voltage detecting circuit 7b must contain a means for correcting and adjusting an offset error and a gain error of the voltage detecting circuit per se. To this end, variable resistors are used. It is difficult to automate the adjustment by the variable resistors. Therefore, intricate and troublesome work is essential at the time of manufacturing and adjusting.
Since the control device for a PWM controlled converter is thus constructed, particularly when the voltage of the 3-phase AC power source 1 drops or is interrupted for a short time by, for example, an instantaneous power interruption, an overcurrent problem arises when the power voltage is returned to the original one.
The problem stated above will be described hereunder.
FIG. 32 shows waveforms of an R-phase power source voltage eR, an R-phase input current instruction signal iR*, and an R-phase input current detecting signal iR. Those waveforms are correspondingly applied to the waveforms of S- and T-phase. Only the waveforms of those signals and voltage of R-phase will be typically described. The following relation holds among the R-phase power source voltage eR, the R-phase input voltage vR of the PWM controlled converter 2, and the R-phase input current detecting signal iR: EQU eR=L (diR/dt).times.VR
In the above expression, L indicates an inductance value of the reactor 3. The resistance value of the reactor 3 is negligible. Therefore, it is left out of consideration here. In a general PWM controlled converter, the voltage drop across the reactors 3 is several to several tens % of the power source voltage eR. Accordingly, the power source voltage eR and the input voltage vR of the PWM controlled converter 2 are substantially in phase.
In a normal operating condition, the R-phase current controller 18b operates so that the R-phase input current detecting signal iR follows the R-phase input current instruction signal iR*, and produces an R-phase control signal SR*. The R-phase current controller 18b PI controls a deviation of the R-phase input current detecting signal iR from the R-phase input current instruction signal iR*. A proportional gain and an integration gain are set at negative values so as to decrease the R-phase control signal SR* when the R-phase input current instruction signal iR* is larger than the R-phase input current detecting signal iR, viz., the current is increased in the positive direction. The amplitude of the R-phase control signal SR is compared with that of a triangle wave carrier signal output from the carrier wave oscillator 21, and the result of the comparison is output in the form of a pulse width modulation signal. The pulse width modulation signal reflects on the R-phase input voltage vR to PWM controlled converter 2.
When the 3-phase AC power source 1 is interrupted by, for example, an instantaneous power interruption, no input current flows. As a result, a difference is caused between the R-phase input current instruction signal iR and the R-phase input current detecting signal iR. An R-phase control signal SR*, which is opposite in direction to the R-phase input current instruction signal iR*, is caused as shown in FIG. 33. Usually, the R-phase input current instruction signal iR* is controlled to be substantially in phase with the R-phase power source voltage eR. When an instantaneous power interruption takes place, a voltage opposite in polarity to the R-phase power source voltage eR is output as the R-phase input voltage vR of the PWM controlled converter 2. Particularly, when the R-phase current controller 18b is designed using the analog control technique to have a high response in its control system, the accumulation of a value of the integrating element of the R-phase current controller 18b increases in the opposite polarity for a short time.
The phase of the output signal of the unit sine wave generator 11 is used as the reference phase of the R-phase input current instruction signal iR*. Usually, the unit sine wave generator 11 is constructed with a circuit having a fixed time constant, and the like. Therefore, even if the 3-phase AC power source 1 is interrupted for a short time, the phase of the power source voltage remains invariable.
When the power source voltage is returned to its original voltage, or the power source is restored from its interruption, a difference between the R-phase power source voltage eR and the R-phase input voltage vR to the PWM controlled converter 2 is large. The large difference of voltage is applied across the reactor 3, so that the R-phase input current detecting signal iR abruptly-increases. The abruptly increased current (referred to as a spike current) possibly causes an overcurrent protecting mechanism to trip. Incidentally, the overcurrent protecting mechanism is generally incorporated into the circuit for protecting the switching elements 201 to 206.
When the input current instruction signal is large, for example, when large power is supplied to the load unit 5, the R- and T-phase current deviations eiR and eiT as the input signals to the R- and T-phase current controllers 18b and 19b are large, and a value of the integration term as one of the component elements of each current controller becomes larger. And a difference between the voltage and the power source voltage of the related phase further grows. Consequently, the spike current caused at the time of the restoring of the power source further increases, which in turn drives the overcurrent protecting mechanism to trip.
The overcurrent problem description, which has been made about the R-phase AC input current control in the converter control device, is correspondingly applied to the same problem in the AC input current control of the other phases.
When the 3-phase AC power source 1 is interrupted for a long time, it is easy to detect such an interruption. When the supply of the power source is interrupted for a short time as when an instantaneous power failure takes place, particularly when the power interruption duration is approximately 1/2 as large as the period of the power source frequency or the power source voltage drop continues for such a short period, it is difficult to detect the power interruption. The conventional device cannot reduce or suppress the spike current, the overcurrent or the like caused when the power source recovers from a state that the voltage of the 3-phase AC power source 1 drops or the supply of the same is interrupted for a short time because of an instantaneous power failure, for example.
The present invention has been made to solve the above problems of the conventional art, and an object of the present invention is to provide a control device for a PWM controlled converter which can satisfactorily control an input current to the PWM controlled converter in a state that when the control device is started up or the power of the load unit abruptly changes, the DC voltage at the output side of the PWM controlled converter drops or a deviation of the actual input current from the input current instruction value as when the input current instruction value abruptly changes.
Another object of the present invention is to provide a control device for a PWM controlled converter which does not need variable resistors for compensating for the offset error and the gain error of the voltage detecting circuits, improves the operability at the time of manufacturing and adjusting, and can easily automate the adjusting operation of the offset and gain errors.
Yet another object of the present invention is to provide a control device for a PWM controlled converter which can satisfactorily control an input current to the PWM controlled converter without causing any spike current when an instantaneous power interruption, for example, takes place, the voltage of the power source drops or the supply of the power source is interrupted for a short time, and the power source is restored to its original state.
Still another object of the present invention is to provide a control device for a PWM controlled converter which can satisfactorily control an input current to the PWM controlled converter without causing any spike current also when the overcurrent protecting mechanism is easily tripped, for example, when the input current instruction value is large, the voltage of the power source drops or the supply of the power source is interrupted for a short time, and the power source is restored to its original state.